This invention is directed to a fastening assembly wherein a nut is doubly locked to an elongated fastener first by frictional engagement of asymmetrical threads and secondly by swaging of portions of the nut into discontinuities on the elongated fastener.
Nuts and bolts are ubiquitously used to attached work pieces together. A common nut and bolt utilizes a common thread to attach the nut to the bolt and clamp the work piece under a clamping load. Such a common thread is generally called a "Unified" thread or an "American Standard" thread. These threads typically have individual threads which have crest and roots formed at about 60.degree. angles. The top of the crests and the bottom of the roots are truncated about a line which is equal to about 1/8 of the pitch of the thread. Similar threads include outdated sharp "V" threads which do not have their crests and roots truncated and the British "Whitworth Standard" threads which utilize a 55.degree. angle between the sides of the threads about the root.
A nut attaching to a bolt by a common thread is subject to being dislodged or worked free from the bolt because of certain environmental conditions. These include vibrations of the work piece on which the bolt and nut are utilized and temperature excursions of the nut and bolt and/or work piece.
The interlock between the nut and the bolt utilizing a common thread concentrates an undue percentage of the load on the first one or two threads with little or no load being carried by the threads which are four or five threads removed from the first thread. Thus, for instance, the first thread can carry upwards of 35% of the load, the second thread 20% of the load, the third thread 15% of the load with the remainder of the load distributed upon the remaining threads. The high concentration of the load on the first engaged thread can lead to bolt failure. This is especially pronounced when soft materials, as for instance, aluminum are utilized for fasteners.
It has been proposed that the undue amount of load concentrated on the first one or two threads occurs because as loading starts on the threads, the male fastener begins to stretch and elongate between the head of the fastener and the first engaged thread. As this happens the first engaged thread starts to deform o deflect. As the clamp load continues to increase the second thread picks up some of the load. However, in order for a significant amount of the load to be transferred to the third, fourth and other ensuing threads, a force high enough to accomplish such ensuing thread loading can cause the first thread to shear or strip. If the first thread strips the load is then concentrated onto the second thread and a chain reaction can occur.
In my prior United States patent, U.S. Pat. No. 4,682,520, entitled MECHANICALLY LOCKABLE FASTENER ASSEMBLY which issued on July 28, 1987 from application Ser. No. 671,823, filed Nov. 15, 1984, and assigned to the same assignee as this application, the entire contents of which are herein incorporated by reference, I describe a fastening system finding great utility in hostile environments. Additionally in that patent I describe other known fasteners which utilized some sort of expedient to fix a nut to a bolt.
The lockable fastener assembly of my U.S. Pat. No. 4,682,520 is very utilitarian and represents a significant advancement in the art over the other fastening systems identified in that patent. In the fastening assembly of that patent, once the work piece is put under a clamping pressure by taking up the nut onto the bolt the nut is permanently fixed to the bolt by swaging material from the nut into grooves formed on the bolt. While the fastening assembly of U.S. Pat. No. 4,682,520 utilizes common threads, irrespective of the loads on the threads the nut material swaged against the bolt permanently locks the nut to the bolt. The swaging of the nut to the bolt additionally serves as a visual indication that the nut is still in place on the bolt.